Method of bonding



United States Patent 3,340,597 METHOD OF BONDING George Ernest Stein, Henrico County, Va., and Frank Leonard Arnold, Fairview Village, Pa., assignors to Reynolds Metals Company, Richmond, Va., a corporation of Delaware No Drawing. Filed June 28, 1963, Ser. No. 291,258 11 Claims. (Cl. 29488) This invention relates to a novel process for the manufacture of a strongly bonded composite of aluminum or aluminum base alloys and stainless steel. More particularly, the invention concerns a novel method of the cladding of aluminum sheet, plate or strip with stainless steel by means of cold or hot rolling.

There has long existed a need for a dependable and economical process of preparing aluminum sheet clad on one or both sides with stainless steel sheet, or conversely, of stainless steel sheet clad on both sides with aluminum or aluminum alloy sheet. The difiiculty of obtaining a strong bond between these two'metals has lOng retarded progress in this field. Methods heretofore proposed in the prior art have included casting of molten aluminum upon steel sheet surfaces, conditioning of the steel surface by heat treatment in an atmosphere of nitrogen, and dipping or spraying of molten aluminum upon an intermediate metal interlayer placed on the steel, as well as hot rolling of metal sheets, and pressure welding.

In the prior art processes, there has existed a general recognition of the importance of the influence of the presence of oxide films upon both the aluminum and the cladding metal upon the strength of the final bond of the clad composite. Accordingly, it was customary to produce a roughened surface both on the steel and upon the aluminum by mechanical means, such as abrasion of the aluminum and base metal surface by wire brushing, grit blasting, sanding, and thelike. However, even where such mechanical conditioning methods were used, it was frequently necessary to employ a cumbersome and costly vacuum sealing step to improve bonding. Moreover, in known processes for bonding aluminum to stainless steel by rolling, it has been the practice to roll the two metals together in such manner that both sheets or plates were at the same temperature, whether cold or hot rolling was employed, followed by reheating of the composite to permit recrystallization of the aluminum to strengthen the bond.

Where both metals to be bonded are at the same temperature, virtually entire dependence for removal of metal oxide layers or films which may interfere with bonding is placed upon prior preparation or conditioning of the metal surfaces, for example, by abrasion. Metal deformation occurring during rolling tends to break up oxide films or layers to a limited extent and thus to enhance metal-tometal contact. In US. Patent 2,753,623 there is proposed the heating of at least one of the metals to be bonded without appreciable oxidation to an elevated temperature having as its upper limit the temperature at which the metal would pull apart when worked, or at which brittle compounds or liquid phase material would form at the surface. However, this patent suggests that if the temperature of one of the metals is below approximately 500 F., additional surface preparation to remove barrier films from that metal should be used.

In accordance with the present invention, it was found, surprisingly and unexpectedly, that aluminum or aluminum base alloys can be successfully bonded to stainless steel by rolling to achieve a bond strength heretofore unattainable, when the aluminum or aluminum base alloy is maintained during the rolling at a temperature substantially higher than the temperature of the stainless steel, specifically at a temperature at least 450 F. higher.

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runowing the bonding step, the process of the invention employs a thermal post-treatment, comprising a reheating bond strengthening operation which also serves to anneal the aluminum or aluminum base alloy.

The aluminum which may be employed for cladding purposes in accordance with the invention includes not only pure aluminum, e.g. of purity 99% or greater, but also both non-heat-treatable and heat-treatable alloys. The common or non-heat-treatable alloys which are suitable are those containing elements which remain substantially in solid solution or which form constituents which are insoluble, and include, for example, the high purity alloys (99.75%, 99.5%), wrought aluminum greater than 99.0%, and the alloys 1100, 3003, 3004, 5005, 5005, 5050, 5052, 5056, and 5154. Among heat-treatable aluminum alloys which can be used are included magnesium-containing alloys such as 6061 and 6062, and zinc-containing alloys such as 7075. The numerical designations given are those of the Aluminum Association System. Where a magnesium-containing alloy is used, it is desirable to clad the core alloy on the side to be bonded with a thin layer, e.g. a 10% thickness of an aluminum base alloy such as type 1100' or higher purity aluminum alloy, preferably an alloy not containing magnesium. This avoids undesirable oxide formation when heating prior to the bonding step.

As employed in this application, the term stainless steel includes ferrous metal alloys containing a substantial amount of chromium or of chromium and nickel having high strength, hardness, and toughness, combined with good ductility, such as, for example, 18% chromium-8% nickel, types 302, 304, and 305.

Stainless steel-bonded aluminum is of importance for the manufacture of cooking utensils, automotive trim, food processing equipment, storage tanks, and highway tankers and trailers. It provides the advantages of both metals. In connection with cooking utensils, such as pans, it permits employing the stainless steel on the cooking side of the vessel, while simultaneously allowing utilization of the heat transmission qualities of the aluminum on the heating or fore side of the pan. Where the composite is employed in transportation equipment and the like, an important weight saving is achieved by reason of the low density of the aluminum, coupled with economies resulting from saving of costly stainless steel.

In accordance with the invention the aluminum or aluminum base alloy may be clad on one or both sides with stainless steel. Or the aluminum may form the center of a sandwich between two layers of stainless steel, if desired.

In carrying out the process of the invention, the surface of the aluminum or aluminum base alloy, which may be previously annealed if desired, is degreased, and is then roughened by wire brushing, sanding, grinding, or like methods, in order to remove any film or thin layer of aluminum oxide present which would interfere with bonding. If desired, a chemical dip may also be employed.

An advantage of the process of the present invention is that, owing to the maintenance of the temperature differential between the heated aluminum or aluminum alloy, and the stainless steel, which is held at the lower temperature, the stainless steel may be bonded without preparation of its surface, and with very little appreciable loss of bond strength in the final composite. However, if desired, the surface of the stainless steel may be conditioned or roughened by mechanical or abrasion methods, such as wire brushing, and the like. The stainless steel is also first subjected to degreasing by any conventional method, and may also be annealed if desired.

The prepared metal sheets, strips or plates are then brought to their respective temperatures, so that there is established a temperature differential between the aluminum or aluminum base alloy whereby the latter is at a temperature at least 450 F. above the temperature of the stainless steel. The working upper limit of temperature for the aluminum member will ordinarily range between about 600 F. and about 1200 F., and preferably between about 900 F. and 1100 F. The stainless steel is maintained at ambient or room temperature, or at a temperature higher than room temperature up to about 450 F., the temperature differential between the aluminumand the stainless steel being at least 450 F. In accordance with preferred procedure, the aluminum or aluminum base alloy is heated to about 1000 F. and rolled against the stainless steel sheet at room temperature.

After attaining their respective temperatures, the metals are placed in contact and pressed together by rolling or other suitable means, employing a force such that the aluminum is reduced from about 5% to about 90%, and preferably about 20% to about 40%, of its original thickness in a single pass. The amount of reduction necessary for bonding is dependent upon the temperature of the aluminum or aluminum base alloy, its surface preparation, and the surface condition of the stainless steel. In the process of reduction the stainless steel is also reduced, but to a lesser extent than the aluminum. Bonding in this manner results in a deformation of the two metals whereby, owing to the temperature differential present, oxide films or layers are ruptured to produce excellent metalto-metal contact and firm preliminary bonding. The extent of this bonding depends upon the type of aluminum or aluminum alloy employed, its surface treatment, its preheat temperature, the surface of the stainless steel, and the amount of reduction employed in making the bond.

Following the preliminary bonding step, the process of the invention employs a reheating bond strengthening operation which also serves to anneal the aluminum base alloy. This is performed by reheating the bonded composite in a furnace at a temperature between about 500 F. and about 975 F., preferably between about 750 F. and about 950 F., for at least 5 minutes. Generally a reheating temperature of 850 F. and a heating of about 30 minutes effectively strengthen the bond produced in the rolling operation. The rate of heating, temperature, and time of heating are not critical and may be varied within wide limits. During this treatment, the bond strength is considerably increased, and no embrittlement of the metal occurs.

In accordance with a modification of the invention, the stainless steel bonding surface may be covered with a tightly clinging sheet of aluminum foil, being interleaved in the coil of stainless steel prior to rolling as by feeding in from a separate coil. The aluminum leaf provides an improved as-ro-lled bond, and upon reheating, a still higher bond strength is attained. Thus, excellent bonds were obtained when the stainless steel surface was covered with a tightly clinging sheet of aluminum foil of 99.7% purity, 0.003 thick. In this manner an excellent bond can be obtained when the stainless steel surface is in contact with the aluminum or aluminum base alloy, the aluminum being at an elevated temperature, after the aluminum has been given enough reduction to rupture the oxide surface.

The success of the bonding operation where stainless steel at room temperature is rolled against aluminum at a temperature in the vicinity of 900 F. depends upon keeping the stainless steel from heating above about 400 F. until it is actually bonded. To measure thermal characteristics during roll bonding, thermocouples may be inserted in the aluminum and stainless steel to be bonded and their cooling and heating rates observed. Thus, where rolling together 304 grade stainless steel at room temperature, and 3003 aluminum alloy, at 900 F., the aluminum cooled about 60 F., while the stainless steel temperature rose to about 360 F. immediately before contacting the rolls. The use of aluminum foil wrapping on the stainless steel serves to control the temperature rise of the latter.

The strength of the preliminary bond obtained by the method of the invention is indicated by a stripping test which permits a quantitative measurement of bond strength. In accordance with this test a one-inch wide, gauge thickness strip is cut from the composite perpendicular to the rolling direction. The stainless steel and aluminum components are each gripped and pulled at a constant speed so that the load necessary to strip is a measure of bond strength. When stripped in a standard tensile strength testing machine, the typical load necessary ranged from 7 to 45 lbs. in the as-rolled condition. After the reheat on bond strengthening treatment, the stripping load was 75 to lbs., as shown by the data in Table 1:

TABLE I.STRIPPING RESISTANCE The following examples serve to illustrate the performance of the novel process of the invention, but are not to be considered as limiting:

Example 1 A composite was prepared from sheets of annealed aluminum base alloy 3003, 0.070 inch thick, and stainless steel type 304, 0.015 inch thick. Both pieces were rectangles 4" x 8" in size, and were first degreased and subjected to wire brushing to condition and roughen their surfaces. The aluminum alloy piece was then heated to 900 F. and rolled against the stainless steel piece which was at room temperature in a 5" x 8", two high rolling mill, using a roll setting of 0.024 inch, in a single pass. After rolling the thickness of the aluminum alloy was 0.042, representing a reduction of about 40%, while the stainless steel piece had a thickness of 0.012 inch, representing a reduction of about 20%. The bond strength was 7 pounds per inch to strip. The composite was reheated at 850 F. for 15 minutes, whereupon the bond strength increased to 150 pounds per inch to strip.

Example 2 A composite was prepared using pieces of a size 12" x 12", respectively, of annealed aluminum alloy 3003, 0.080" thick and stainless steel type 304, 0.015 thick, both pieces being subjected to degreasing and wire brushing for preconditioning. The aluminum alloy piece was heated to 900 F. and rolled against the stainless steel piece which was at room temperature, in a four high rolling mill, having 8 /2" X 20" work rolls, setting 0.032". After rolling the thickness of the aluminum alloy was 0.063", representing a 21% reduction, while the thickness of the stainless steel layer was 0.0135, representing a 13% reduction. The composite was reheated for 30 minutes at 850 F. and was readily drawn to form a pan.

Example 3 A composite was prepared which was a sandwich of aluminum base alloy between two stainless steel pieces, all pieces being of a size 12" x 12". The pieces of stainless steel, each 0.015" thick, were first degreased and wire brushed. The aluminum alloy piece, 0.090" thick, was degreased and wire brushed on both sides. The aluminum piece was annealed alloy 1100, the stanless steel was type v The composite was reheated for 30 minutes at 850 F. and then readily drawn to form a pan.

Example 4 A composite was prepared of stainless steel between two aluminum base alloy pieces, all pieces being of size 4 X 8". The stainless steel was type 304, preconditioned by degreasing and wire brushing, and was 0.015 thick. Both aluminum alloy pieces were of annealed all-oy 3004, 0.067 thick with cladding of 3A aluminum on both sides, and were degreased and wire brushed. Both aluminum alloy pieces were heated to 900- F. in a furnace during a period of minutes and rolled in a 5" x 8 two high rolling mill, setting 0.023", against the stainless steel piece which was at room temperature. After rolling, the thickness of the aluminum alloy pieces was 0.041 each, a reduction of 39%, while the thickness of the stainless steel was 0.0125, a 21% reduction. The bond strength of this composite was 15 pounds per inch to strip. The composite was reheated 15 minutes at 900 F., whereupon the bonding strength increased to 75 pounds per inch to strip.

Example 5 A composite was prepared from pieces of annealed aluminum base alloy 3003 and stainless steel type 304, each piece 4" x 8". The aluminum alloy piece was 0.069" thick and was degreased and wire brushed. The stainless steel piece was 0.015" thick and after degreasing and brushing was wrapped with 99.9% aluminum foil 0.003" thick.

The aluminum alloy was heated to 900 F. and rolled in a 5" X 8", two high rolling mill, setting 0.015", against the stainless steel piece which was at room temperature. The composite was immediately put back into the furnace at the same temperature for 5 minutes and then rerolled with the mill setting 0.006". After the second rolling, the thickness of the aluminum alloy was 0.037, a 49% reduction, while the thickness of the stainless steel was 0.0135", a 14.5% reduction.

What is claimed is:

1. Method of bonding aluminum or an aluminum base alloy to stainless steel to form a composite, which comprises treating the bonding surface of the aluminous metal to effect removal of aluminum oxide film, heating the aluminous metal to a temperature between about 600 F. and about 1200" F., and rolling said treated surface against the stainless steel while maintaining the stainless steel at a temperature between ambient temperature and about 450 F., the temperature differential between the metals being at least 450 F.

2. Method of bonding aluminum or an aluminum base alloy to stainless steel to form a composite, which comprises treating the bonding surface of the aluminous metal to effect removal of aluminum oxide film, heating the aluminous metal to a temperature between about 900 F. and about 1100 F., and rolling said treated surface against the stainless steel while maintaining the stainless steel at substantially ambient temperature.

3. The method of claim 1 in which said treatment of the aluminous metal surface includes mechanical roughemng.

4. The method of claim 1, in which the composite obtained by rolling is bond strengthened by heating at a temperature between about 500 F. and about 975 F.

5. The method of claim 4 in which the composite is subjected to a drawing operation after heat treatment.

6. The method of claim 1 in which two webs of aluminum base alloy are bonded to opposite sides of a core of stainless steel.

7. The method of claim 1 in which two Webs of stainless steel are bonded to opposite sides of a core of aluminum base alloy.

8. The method of claim 1 in which the rolling pressure is sufiicient to cause a reduction in thickness of the aluminous metal of from about 5% to about of its original thickness. 4

9. The method of claim 1 in which the aluminum base alloy surface is clad with high purity aluminum prior to rolling.

10. The method of claim 1 in which the stainless steel bonding surface is covered with aluminum foil prior to rolling.

11. The method of claim 1 in which the initial thickness of the aluminous metal is approximately four times the thickness of the stainless steel.

References Cited UNITED STATES PATENTS 2,908,073 10/1959 Dulin 29494 X 3,078,563 2/1963 Gould et a1 29497.5 X 3,093,885 6/1963 Morrison et al. 29-l96.2 X 3,132,418 4/1964 Fulford 29497.5 X 3,173,202 3/1965 Farber 29497.5 X 3,210,840 10/1965 Ularn 29497.5 X

JOHN F. CAMPBELL, Primary Examiner, DROPKIN, Assistant Examiner. 

1. METHOD OF BONDING ALUMINUM OR AN ALUMINUM BASE ALLOY TO STAINLESS STEEL TO FORM A COMPOSITE, WHICH COMPRISES TREATING THE BONDING SURFACE OF THE ALUMINUM METAL TO EFFECT REMOVEL OF ALUMINUM OXIDE FILM, HEATING THE ALUMINOUS METAL TO A TEMPERATURE BETWEEN ABOUT 600*F. AND ABOUT 1200*F., AND ROLLING SAID TREATING SURFACE AGAINST THE STAINLESS STEEL WITH MAINTAINING THE STAINLESS STEEL AT A TEMPERATURE BETWEEN AMBIENT TEMPERATURE AND ABOUT 450*F., THE TEMPERATURE DIFFERENTIAL BETWEEN THE METALS BEING AT LEAST 450*F. 